Field of the Invention
The invention is related to power steering systems for automotive vehicles and in particular to electrically assisted power steering systems.
Hydraulic power steering systems for automotive vehicles have enjoyed wide acceptance by the using public and are available on almost every type of commercially available vehicle. With the down sizing of automobiles and the use of smaller four cylinder engines for economy reasons, the power requirements of the hydraulic pump for power steering systems have become an appreciable portion of the total available power. This is not only true during parking or slow motion maneuvers when the engine is idling but also at highway speed where the hydraulic pump absorbs a portion of the available engine power in parasitic losses such as just spinning the pump. To prevent the engine from stalling due to the increased load on the idling engine during low speed maneuvers, it has been necessary to increase the engines idle speed. Unfortunately the increased idle speed is only required when the low speed steering maneuvers are being executed. At all other times it is unnecessary. Therefore increasing the idle speed to accommodate the use of the hydraulic power steering during low speed steering maneuvers is counterproductive to the economical advantages of the small engines.
Electrical power assisted steering systems, which use the reserve power of the battery during low speed maneuvers could overcome the disadvantages of the hydraulic power steering systems in the smaller vehicles. The concept of using electrical power to assist steering is not new. Hepner in U.S. Pat. No. 3,191,109 and Turible in U.S. Pat. No. 3,351,152 disclose electrical power assisted steering systems having a segmented steering column interconnected by an electrically driven gearing arrangement. Turible also discloses disabling the power assisted steering at speeds above a predetermined low speed to give the operator a normal feel of the road. The power steering arrangement taught by Hepner and Turible are unsatisfactory because the reverse torque generated by the electrically driven gearing arrangement is transmitted back to the operator through the steering wheel.
This disadvantage was overcome by Goodacre et al in U.S. Pat. No. 3,534,623. In the assisted steering system disclosed by Goodacre et al, the two sections of the steering column are directly coupled by a gear train. The gear train responds to a torque above a predetermined level to energize an electric motor coupled to the section of the steering column connected steering gear box. Goodacre et al's system has the disadvantage that it uses a high gear ration between the steering column and the electric motor and requires that the steering wheel must be manually turned to return the vehicle's wheels to their neutral position with the wheels aligned along a straight path. Also the power assist is either "on" or "off" giving the drive little or no feel of the road at the higher speeds.
This problem was overcome by the use of torque responsive manual clutches such as taught by Steinmann in U.S. Pat. No. 3,893,534, Bayle in U.S. Pat. No. 3,983,953 and Adams in U.S. Pat. No. 4,223,254. Deininger et al in U.S. Pat. No. 4,241,804 teaches the same basic concept but uses a counter-rotating clutch mechanism with a non-reversible motor.
The concept of controlling the electrical power applied to the electric motor as a function of the torque applied to the steering wheel is also taught by Bayle, cited above. Bayle discloses a torque sensor which generates a torque signal proportional to the angular displacement between the two sections of the steering column and a pair of switches which generate direction signals indicative of the direction of the applied torque. A computer computes a current applied to the electric motor which is a functon of torque applied to the steering wheel. In alternate embodiments taught by Bayle, the computed electrical power applied to the motor is further modified by an input indicative of the engine speed to improve driveability at nominal road speeds above a predetermined level. Further Bayle teaches the use of an electric clutch to disengage the motor from the steering wheel to eliminate the requirement for the operator to manually return the wheels to their neutral position.
The invention is an electrical power assisted steering system which eliminates both the need for a clutch to disengage the steering column from the electric motor during the return of the wheels to their neutral position and the need for an input indicative of the engine speed for normal and high speed operation.